Simple Circuit Experiment | Fun and easyBy Robolab Technologies In Atal Tinkering Labs ATL
The aluminum foil is the conductive path that the battery energy follows to light up the bulb.
There are a couple of other ideas that you could do to change up the project a little bit and see what happens.
You could try a different size battery, stack a couple of batteries together or have the aluminum foil make contact with different things.Try to make more complex circuits. See how many light bulbs will light up with one 9 volt battery.